Andesite

Andesite is a fine-grained volcanic rock with intermediate silica content, composed mainly of plagioclase feldspar. It commonly forms when magma erupts at convergent plate boundaries.

Plagioclase: A group of feldspar minerals forming a continuous solid-solution series from albite to anorthite. Plagioclase is typically andesite’s main mineral constituent.
Subduction: The descent of one tectonic plate beneath another into the mantle. Water released from the sinking slab helps generate many andesite-producing magmas.
Basalt: A fine-grained volcanic rock low in silica and rich in iron and magnesium. Basalt is generally more mafic and less silica-rich than andesite.
Mount St. Helens: An active stratovolcano in Washington State, United States, known for its 1980 eruption. Its eruptions produced abundant andesitic and dacitic lava and tephra.
Silica content: The proportion of silicon dioxide in a rock or magma, used to compare igneous compositions. Andesite’s intermediate silica content places it between basalt and dacite.
Flux melting: Melting caused when added volatile substances lower the melting temperature of rock. Water from subducted oceanic crust promotes melting in the mantle wedge.
Dacite: A volcanic rock richer in silica than andesite, commonly containing plagioclase and quartz. Dacite marks the more silica-rich side of andesite’s compositional range.
Mount Fuji: A stratovolcano on Japan’s Honshu island and the country’s highest mountain. Its volcanic products include andesite, illustrating arc volcanism in Japan.
Igneous rock: Rock formed when molten material cools and solidifies. Andesite belongs to the broad family of rocks crystallized from magma or lava.
Fractional crystallization: Magma evolution in which early-formed crystals separate from the remaining melt. Crystal removal can shift basaltic magma toward andesitic composition.

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